The present invention relates to improvements in rotary pile boring tools. In particular, the present invention relates to a boring xe2x80x98bucketsxe2x80x99 for collecting spoil generated during rotary piling operations.
Boring buckets are commonly used by the piling industry when rotary piles are bored under a support fluid, for example bentonite, polymer and even water. Buckets are most commonly used when the strata is non-cohesive, or inter-bedded with cohesive layers.
Typically a boring bucket consists of a cylindrical vessel with a base. It is usually connected to the rotary drive unit by means of a kelly bar. The base of the bucket may be flat or shaped like an inverted cone.
The base has openings (generally two or more) and may be fitted with some sort of soil cutting means, such as xe2x80x9cteethxe2x80x9d or xe2x80x9cpicksxe2x80x9d, which project below the base of the bucket. During the piling operation, the tool is rotated, thereby causing the teeth to cut into the underlying soil. The soil which is generated then enters the bucket through a number of holes or openings in the base. Generally the base is hingeably mounted to the bottom of the vessel and there may also be provided a mechanism which locks the base to form a secure closure at the bottom of the vessel during boring.
After the boring bucket has been withdrawn from the bore, the mechanism can be actuated to allow the base to swing open, thereby discharging the spoil that was excavated.
One type of bucket in common use is often fitted with steel or rubber flaps which are positioned above the openings in the base. These flaps are intended to allow the passage of soil into the bucket and also serve to retain the soil during the withdrawal of the bucket from the bore.
The diameter of the barrel of the bucket is often smaller than the design diameter of the required pile. In order to address this, it is known for these types of bucket to be fitted with xe2x80x9cside-cuttersxe2x80x9d or teeth which are positioned at the lower end of the barrel. These teeth are oriented with the cutting blades essentially vertical. As the tool is rotated the surrounding soil is cut away from the side of the bore. To allow the soil excavated by the side cutters to enter the bucket, openings positioned just ahead of the side cutters are provided in the barrel. The side cutters are usually fixed such that the overall dimension across the cutters is equal to the design diameter of the pile.
However, it has been found that this bucket design suffers from the disadvantage that a considerable amount of soil can actually be lost through the openings positioned ahead of the side cutters. This not only reduces productivity, but also contaminates the support fluid.
Another type of boring bucket which is known in the art is a xe2x80x9cswivel-basexe2x80x9d, xe2x80x9crotatable-basexe2x80x9d or xe2x80x9cguillotinexe2x80x9d bucket. This type of bucket also has a fixed base, often in the form of a hinged baseplate similar to that described above. Often the barrel of these types of buckets is a truncated cone.
In addition, a second baseplate comprising a rotatable-base, is fixed to the underside of the fixed baseplate, by means of a short co-axial shaft. This arrangement allows the rotatable base to rotate axially with respect to the fixed baseplate.
During the digging process, the tool and bucket are rotated in a first direction which will be referred to from now on as the xe2x80x9cdigging directionxe2x80x9d. When the bucket is rotated in the digging direction, the swivel base reacts against the bore, and pivots about a co-axial shaft. This action allows openings in the swivel base to be aligned with openings on the fixed base above, so that the soil generated during digging can pass into the bucket. Once the bucket is full, it can be rotated in the opposite direction, thereby dis-aligning the openings to contain the collected soil within the vessel.
In order to excavate the bore hole, teeth or picks are provided on the underside of the swivel base so that when the tool is rotated, the blades provided on the teeth cut into the soil below. The outer-most teeth (which have their cutting blades oriented essentially horizontally) tend to cut a scrolled groove in the wall of the pile bore. The blades of the teeth are essentially horizontal and the teeth which are closest to the bore surface are usually disposed at an incline and positioned such that the dimension between opposing teeth is equal to the design diameter of the resultant pile.
However, it has been experienced by operators using swivel base buckets that, due to the absence of side cutters and side openings, the full width of the required pile diameter is not reliably excavated by the blades provided below the base. Clearly, in such cases where the diameter of the resultant pile shaft is incorrect, a number of problems will occur. For example, if the diameter of the pile shaft is too small, there can be a risk of reduced shaft adhesion between the pile and the surrounding soil. Furthermore, this may also result in inadequate cover to the reinforcing steel.
To address this problem it has been known to fit one or more reamer blades at the head of the bucket, to ream the bore to the appropriate diameter. However, the use of such a reamer suffers from a number of drawbacks. For example, due to the positioning of the reamer at the head of the bucket, the part of the bore which is below the reamer is not of the correct diameter. Furthermore, the reamer blades have been found to catch on the head of the temporary casing, thereby slowing production. In addition, soil reamed during the boring process commonly falls into the annular gap around the barrel of the bucket. As a consequence, the drilling fluid may become very contaminated.
Accordingly, the present invention seeks to provide a boring apparatus of the type comprising a hollow vessel with a rotatable base, which will excavate the bore to the required design diameter, and which will retain soil excavated from the side of the bore within the hollow vessel.
According to the present invention, there is provided a boring apparatus comprising a substantially hollow vessel having a fixed base and a rotatable base, characterised in that the rotatable base is provided, at its circumference, with at least one side cutter, the side cutter being rotatable with the rotatable base from a first position, in which the at least one side cutter serves to cover a respective opening provided in the vessel wall, to a second position, in which the opening in the vessel wall is exposed.
In contrast to known boring buckets of the fixed base type, in which the side cutters are rigidly fixed to the vessel of the bucket relative to openings provided in the vessel wall, in the present invention the side cutters will rotate relative to the vessel wall during the drilling operation. An advantage of the present invention is therefore not only that the side cutters serve to excavate the side of the bore, but also that the openings are disposed in the vessel wall such that the side cutters effectively cover the openings when the digging action has ceased, thereby preventing excavated ground material from falling out of the vessel.
Preferably the at least one side cutter is in the form of a support arm (or up-stand) provided with a blade, the side cutter serving to cut soil from the edge of the bore hole to allow the outer diameter of the bore to be accurately achieved. The side cutter is preferably arcuate in form, the support arm extending around a part of the circumference of the cylindrical vessel.
Preferably two side cutters are provided which are located on opposite sides of the vessel. The overall dimension between opposing side-cutting teeth (or upstands) is equal to the design diameter of the pile.
Preferably, the fixed baseplate and the rotatable base plate are both provided with openings therein, which are configured so that when the rotatable base member reacts against the bore during a digging operation, the openings in the two base members come into alignment thereby allowing soil to enter into the vessel. Once the vessel is full, the rotational motion ceases, and the base member rotates once more with respect to the vessel body thereby dis-aligning the openings.
The vessel body may be cylindrical or tapered as shown in the accompanying drawings. The lower end of the vessel may be of uniform diameter or may be tapered.